The use of LEDs as light sources is becoming more widespread for the numerous advantages compared to traditional incandescent and fluorescent lamps, mainly as regards consumption, versatility and flexibility of control, which allows the most varied effects to be achieved. The LEDs are frequently used for the lighting of indoor and outdoor environments. Among the many uses, for example, LEDs can be advantageously exploited for the lighting of streets, monuments and architectural structures, in the entertainment industry for the implementation of stage lighting (both for effects and for manufacturing Wash Light type projectors). In the interior, LEDs are also often preferred for home lighting.
The lighting quality provided by the sources is crucial in many applications. In particular, it is preferred that the colour rendering of the sources be as natural as possible, as this strongly affects perception. The colour rendering is normally measured by a parameter, the so-called Colour Rendering Index CRI or Ra, which is determined according to a standardised procedure and provides objective evidence that the light from a source approximates the radiation of a black body at a given colour temperature.
The currently available LED sources are not yet fully satisfactory in terms of colour rendering. To improve the colour rendering, LED lighting modules have been proposed based on the combination of LED sources with different emission spectra. Nevertheless, the colour rendering index is not high enough or, at the least, the rendering is satisfactory only in a narrow range of colour temperatures. For example, in the entertainment industry, which requires an excellent colour rendering, the mandatory replacement of tungsten and halogen lamps is a problem perceived in an increasingly urgent way. In fact, the environmental and energy saving standards aim to prevent the use of low-efficiency sources, such as the incandescent lamps, which however have a colour rendering unmatched by other currently available discharge or LED sources.